MICROSTRAIN
IN POLYCRYSTALLINE METALS* N. BROWN? and K. F. LUKENS Jr.? A theory was developed for describing the dependence of microplastic strain on stress and gram size of polycrystalline metals. The microplastic strain is given by y = CpD3(a -Q,~)=/&T,~ where C w l/2. p is the density of sources, u the applied stress, u,, 0 stress to move first dislocation, a shear modulus and D grain size. The theory was checked by measuring the stress-strain curve in the micro region using a capacitance type extensometer. The metal investigated was ingot iron with grain sizes from 44 to 140 p. The agreement between the theoretical equation and the experimental data was excellent not only in the case of the iron but for data on copper and zinc from the investigation of others. The density of sources in ingot iron and copper was 2.5 and 6.9 x lo6 sources/ems, respectively. The range of the microstrain region has been calculated in terms of the amount of microstrain that occurs at the macroscopic yield point. As a useful sidelight it turns out that annealing ingot iron at 650ยฐC after a microstrain completely removes the work hardening so that the specimen returns to its initial state. Consequently, the same specimen may be used repeatedly for tests in the microstrain region. MICRO-DEFORMATION DANS DES METAUX POLYCRISTALLINS Les auteurs ont developpe une theorie decrivant la dependance de la micro-deformation plastique, de la tension et de la dimension des grains dans un metal polycristallin. La micro-deformation plastique est don&e par: y = CpD3(cr -u~")~/Qu,~ oti C w l/2 p est la densite des sources, u la tension appliquee, u 0ยฐ la tension necessaire pour mouvoir la premiere dislocation, (7 le module de cisaillement et D la dimension du grain. Cette theorie est confirmee par la determination des courbes tension/deformation dans des petite8 regions en utilisant un extensometre type capacitance. Le metal utilise Btait un lingotin de fer dont les dimensions des grains variaient de 44 a 140 ,u. La concordance entre l'equation theorique et les resultats experimentaux s est revelee excellente non seulement dans le cas du fer mais aussi grace It des resultats experimentaux obtenus par d'autres chercheurs dans le cas du cuivre et du zinc. La densite des sources dans le fer et dans le cuivre Btait respectivement 2, 5 et 6,9 x IO" sources/cm3.